1. FIELD OF THE INVENTION
The present invention is directed to an apparatus and process for measuring changes in the effect of tidal forces at a given location on earth. More particularly, the present invention is directed to an apparatus and process which measures changes in the effects of tidal forces in conjunction with measuring leakage of liquids from storage tanks (such as underground fuel tanks used in filling stations) for providing a corrective factor to be considered in the measurements of such leakage.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Leakage of hazardous liquids, such as hydrocarbon fuels, from storage tanks into the environment represents a significant and ubiquitous hazard. Specifically, leakage of fuel from underground fuel tanks of filling stations in the United States and elsewhere has been of great environmental concern, and the subject of laws and regulations which, generally speaking, define the upper limit of permissible leakage at 0.05 gallons per hour.
Those who are familiar with the state of the art know that measuring leakage at a rate of 0.05 gallons per hour from an underground tank of 10,000 gallons or larger capacity, is a very difficult task. It is said sometimes, that the difficulty of this measurement is comparable to measuring the loss (or addition) of a single drop of liquid in a bucket full of the liquid.
As is well known, there are many factors which contribute to the difficulty of measuring leakage from underground fuel storage tanks of fillings stations (and the like) with the above-noted legally required degree of accuracy. Temperature effects are one major cause of difficulty, because they result in contraction or expansion of the fuel in the tank, which, when measured as apparent "leakage", may well be greater than the legally permissible limit of 0.05 gallon per hour.
Another difficulty is that the walls of cylindrical tanks are curved so that the fuel depth-versus-volume relationship in the tank is not linear. When an attempt is made to overcome this difficulty by filling the tank to the brim, up to the vertically disposed cylindrical fill pipe, then another cause for error arises. This error is caused by the unusually large hydrostatic pressure in the tank, which causes unpredictable expansion of the tank walls.
A currently pending patent application, filed on Mar. 16, 1987 and having Serial No. 026,113 in the United States Patent and Trademark Office, and assigned to the same assignee as the present application, describes a process whereby specific steps are taken to measure and calculate the effect of temperature induced contraction or expansion of fuel in the tank. U.S. Pat. No. 4,646,560 also describes an apparatus which compensates for the temperature effects in the tank.
For further description of apparatus relating to the measuring of leakage of liquid from storage tanks, reference is made to U.S. Pat. Nos. 4,397,183; 4,373,815; 4,474,054; 4,561,291; 4,672,842; 3,841,146; 3,460,386; 4,604,893; 3,889,523; and to a publication by the United States Environmental Protection Agency, titled UNDERGROUND TANK LEAK DETECTION METHOD: A STATE-OF-THE-ART REVIEW, dated January, 1986.
Whereas the above-noted and other devices attempt to monitor the level of fuel in the tank by accurate monitoring of the position of a float in the tank, and whereas some of the above-noted devices also attempt to measure or compensate for temperature effects, the present applicant is unaware of prior art devices which measure or compensate for tidal effects in the tanks. Nevertheless, it has been discovered in accordance with the present invention that tidal effects surprisingly do occur in underground storage tanks, and that such tidal effects can seriously interfere with the measurements of leakage from the tanks. Accordingly, there is a need in the art for a process and apparatus which measures and compensates for the tidal effects. The present invention provides such process and apparatus.